The conventional filter of the ladder type has a box-like casing 1 made of an insulating material as shown in FIG. 7. One side face of the casing 1 is opened to form an aperture 1a through which elements and devices are put into the casing 1.
As is shown in FIG. 8, a predetermined number of piezoelectric devices 2, a plurality of terminal plates 3 and, if necessary, insulating plates 4 are put into the casing 1 in a predetermined order to form a filter circuit of the ladder type, an equivalent circuit thereof being shown in FIG. 5.
In order to complete the circuit of FIG. 5, as is shown in FIG. 8, a pair of terminal plates 3 inserted between the first and second piezoelectric devices 2 is electrically connected to the terminal plate 3 by a common tip end 5a, which is electrically connected to one electrode of the third piezoelectric device 2. Also, a pair of terminal plates 3 inserted between the third and fourth piezoelectric devices is electrically connected to the terminal plate 3 with a common tip end 5a, which is electrically connected to one electrode of the fifth piezoelectric device 2. Each terminal plate 3 inserted between an end wall 1e of the casing 1 and the first or the fifth piezoelectric device has a terminal tip end 5b passing through the end wall 1e of the casing to form input or output terminals of the filter.
Each terminal plate insulated by the insulating plate 4 relative to the terminal plate having the terminal tip end 5a is electrically connected to an elongated terminal tip 5c by tip end 5d. Each end portion of the terminal tip 5c extends through the end wall 1e of the casing. After the ladder circuit having four terminals is completed in the casing 1, the aperture 1a of the casing 1 is hermetically sealed with a cover plate 6.
In the conventional filter described, each of terminal tips 5a, 5b and 5d is usually internally formed respectively with the corresponding terminal plate 3. Further, there is formed a projection 3a at the center of the terminal plate 3 for contacting the piezoelectric device electrically and resiliently to hold the same between two terminal plates.
The structures of the conventional filter of the ladder type have disadvantages in that it is difficult to assemble a filter automatically by the use of an automatic assembly machine and therefore the filter of this type is not suitable for mass production for the following reasons;
electrodes of the piezoelectric device can be destroyed when projections of the terminal plate are rubbed thereon while putting elements 3 and 4 and devices 2 into the casing of the filter, and
terminal tips 5d must be electrically connected to the terminal tip 5c with solder.